Electric arc apparatus

ABSTRACT

Apparatus for treating a flow of material by an electric arc generates free radicals and/or atoms, and charged species, in ground or excited states. A cathode (13) and an anode (14) are arranged for striking an arc across a preferably annular gap and provide a pathway for flow of material through the gap between the cathode and anode. Each electrode has a region providing a continuous closed path for the anode spot or cathode root respectively to travel around, and means, preferably magnetic means (19), for producing movement of the arc cause the anode spot and cathode root to travel around their paths in the same sense and in substantially the same time. Preferably the paths are circular paths in parallel planes on the anode and cathode respectively, and the length of the arc remains substantially constant during travel of the anode spot and cathode root around the paths. Preferably the cathode (13) comprises a rod having a plane end face with a circumferential edge for striking the arc. Preferably the cathode has a sharp edge for striking the arc and for providing the said continuous closed path for the cathode root.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatus and method for treating aflow of material by an electric arc. The invention is concerned inparticular with a method and apparatus for treating a flow of gaseousmaterial by passing the material through an arc to form an electricalplasma for generating (in a ground or excited state) free radicalsand/or atoms, and charged species, in a jet of gas.

2. Description of the Prior Art

It is known to provide apparatus for producing an electrical plasmasometimes known as a plasma torch, in which a cathode and an anode areprovided within a housing and define an annular space through which agaseous feed stock (a gas, vapour, suspension or other gaseous material)is passed, and a plasma is produced by striking an arc across a gapbetween the cathode and anode. Where the gaseous material passes throughthe gap under pressure and emerges in a stream of material, the plasmatakes the form of a jet extending from the arc region and sometimesreferred to as a plasma jet. In one known form of such a plasma torch,the cathode comprises a rod and the anode comprises a cylindricalstructure with a frusto-conical surface extending inwardly at one end ofthe cylinder, the cathode rod and the anode cylinder being coaxial, withthe tip of the cathode rod positioned inside the region surrounded bythe frusto-conical surface. The end of the cathode is initially pointedor dome-shaped and the arc is struck between the tip of the cathode rodand a region on the frusto-conical surface of the anode. At the centerof the frusto-conical surface of the anode is an aperture leading alongthe axis of the anode cylinder and through the anode for exit of thegaseous material which passes through the region where the arc isstruck. The place on the cathode from which the arc is struck issometimes known as the cathode root and the place on the anode at whichthe arc is struck is sometimes known as the anode spot.

It is known to provide an annular magnet around the cathode-anode axisfor providing a magnetic field in the region of the cathode-anode gap.The magnetic field is arranged in such a manner as to cause the arc tomove in a generally rotary manner around the longitudinal axis of thecathode and anode, and in this movement the anode spot travels in agenerally circular path around the frusto-conical surface of the anode.However the cathode root does not move in any organised manner on thecathode and moves erratically between positions which give the minimumanode to cathode spacing for the arc. In some cases the cathode root mayremain at a position until the cathode surface at that spot issufficiently eroded by the arc to force the arc to jump to anotherposition which again gives a minimum cathode to anode spacing for thearc. Thus in some cases the manner of operation of the arc may be thatthe cathode root makes intermittent, random jumps to new cathode rootpositions after damage has been inflicted on the cathode by erosion atthe previous cathode root position.

The present invention is particularly concerned with the more efficientuse of a plasma jet for generating (in a ground or excited state) freeradicals and/or atoms, and charged species, in a jet of gas.

The most sophisticated plasma jets used in previous work have used apointed cathode (usually of thoriated tungsten) surrounded by a watercooled anode (usually Cu, brass or steel), and were provided with acircumferential solenoid to form an electromagnet. The axial componentof the magnetic field produced by the latter interacted with the radialcomponent of the current, causing the D.C. arc to rotate at high speedabout the central cathode. This helped to heat the gas more uniformly,to generate swirl and turbulence, and to minimise anode erosion.

However, such use of a magnetic field does not cause the cathode root tomove in any organised fashion. Melting tends to occur at the pointed tipof the cathode and the achievement of a compromise between the danger ofrapid consumption of the cathode and the need to maintain the cathoderoot at a sufficient temperature for plentiful electron emission is madedifficult in the case of some gases by electron attachment, by theabsorption of much energy during dissociation, and by other factors. Ithas been the practice in previously known methods to include in the feedstock material an admixture of large amounts of argon or other suitablemonatomic gas additives.

SUMMARY OF THE INVENTION

According to the present invention there is provided apparatus fortreating a flow of material by an electric arc, comprising a cathode andan anode arranged for striking an arc across the gap between the cathodeand the anode and arranged to provide a pathway for flow of materialthrough the gap between the cathode and the anode, in which the anodehas a region providing a continuous closed path for the anode spot totravel around in operation, the cathode has a region providing acontinuous closed path for the cathode root to travel around inoperation, and there is provided means for producing movement of the arcstruck between the cathode and the anode such that the anode spot andthe cathode root travel around their respective continuous closed pathsin the same sense and in substantially the same time.

By the terms cathode and anode is meant electrodes which in operationare intended to act as cathode and anode respectively when connected toan appropriate source of unidirectional voltage.

The terms anode spot and cathode root mean the places on the anode andcathode respectively between which the path of the arc is struck at anyparticular instant of time.

It is to be appreciated that as the anode spot and cathode root progressrepeatedly around their respective paths, there may occur for anyparticular corresponding pair of complete progressions slight variationin the times taken, and also that during such progressions the travelalong one path may marginally advance or retard relative to the travelalong the other paths. However it will normally be preferred that theanode spot and cathode root travel together in regular manner, as nearlyas possible in correlation with each other, and at rates of travel suchthat the times of completed progressions are the same for correspondingprogressions around the two paths.

Furthermore it may occur that the times taken for successive completedprogressions around the respective paths may vary during a number ofrepeated progressions. That is to say that the movements of the anodespot and cathode root around their respective paths are not necessarilyaccurately and reproducible periodic movements in all embodiments of theinvention. However it is much preferred that the arc moving means issuch that in operation the movements of the anode spot and cathode rootaround their respective paths are periodic movements.

Similarly, it is to be appreciated that in some embodiments of theinvention it may be desirable for the distance between the anode spotand the cathode root to vary during progression of the anode spot andcathode root around their respective paths (either due to variations inthe spacing between the paths or due to variations in the rates oftravel of the anode spot and cathode root) with consequent change of arclength. However, it is much preferred that the shape of the cathode andthe anode and the arrangement of the arc moving means are such that inoperation the length of the arc remains substantially constant duringtravel of the anode spot and cathode root around their respective paths.

It is preferred that the said continuous closed paths of the cathoderoot and anode spot lie in respective, spaced-apart parallel planes, andit is also preferred that the said continuous closed paths are circularpaths. Most preferably the circular paths are arranged with theircenters lying on a common axis which most preferably is arranged to beperpendicular to spaced-apart parallel planes in which the circularpaths lie.

It is to be appreciated that in some embodiments the paths may beelliptical, or indeed may follow other closed continuous configurations.The paths are preferably arranged to lead around a common axis, andwhere the paths lie in parallel, spaced-apart planes, it is preferredthat the common axis of the paths is perpendicular to the parallel,spaced-apart planes.

It is also preferred that the cathode has an elongated form and has ageneral longitudinal axis. In such cases, it is preferred that thegeneral longitudinal axis of the cathode should be aligned along thesaid common axis around which the closed continuous paths preferablylead.

In a particularly preferred form of the invention, the cathode comprisesa cylindrical body having, at an end thereof adjacent to the anode, aplane end face perpendicular to a longitudinal axis of the body, thesaid continuous closed path of the cathode root being provided aroundthe perimeter of the plane end face of the cylindrical body. It has beenfound convenient for such a cathode to comprise a rod having a diameterin the range 1 to 5 mm.

It is a particular preferred feature of the invention in some aspectsfor the cathode to have an edge for striking the arc (preferablyextending along a continuous closed configuration) for providing thesaid continuous closed path for the cathode root. Thus where the cathodecomprises a cylindrical body with a plane end face, the said edge isconveniently formed around the perimeter of the plane end face, andcomprises an edge having an angle of 90°. It will be appreciated that inother arrangements the edge may have an angle greater or lesser than90°, for example by comprising a bevelled edge, or a sharp acute edgeformed for example in an outstanding flange or lip. In general thecriteria determining the sharpness of the edge are that the edge shallbe sufficiently sharp to concentrate the lines of the electric field forstriking the arc when in operation a potential difference is appliedacross the cathode-anode gap, and the edge shall have an anglesufficiently great to provide adequate conduction of heat away from theedge while the arc is being struck.

In accordance with another preferred feature of the invention, theanode-cathode gap may be an annular gap, and the anode may have anaperture through the anode coaxial with the annular gap for passage ofmaterial which has passed through the said annular gap. Convenientlywhere the cathode has an elongated form and has a general longitudinalaxis, the anode aperture is aligned along the longitudinal axis of thecathode.

Where an aperture is provided through the anode, the anode is preferablyshaped to provide in the region of the cathode an entrance to theaperture having a decreasing cross-section and to provide at the otherend of the aperture an exit having an increasing cross-section, thearrangement being adapted to provide in operation an expansion ofmaterial passing out from the anode aperture.

Conveniently the anode has facing the cathode an inwardly tapering,frusto-conical surface coaxial with and leading to the aperture throughthe anode, the arrangement being such that in operation the saidcontinuous closed path of the anode spot lies in the frusto-conicalsurface of the anode. The semi angle of the frust-conical surface ispreferably in the range 30° to 50°, and most preferably is substantially45°. Conveniently the anode has a further frusto-conical surface formedon a side of the anode remote from the cathode and coaxial with theaperture through the anode. The semi-angle of the further frusto-conicalsurface is preferably in the range 50° to 70°, where the pressuredifference is sufficient for sonic flow, most preferably substantially60°. The arrangements of the anode in connection with the aperturetherethrough may in appropriate circumstances be arranged so that inoperation gaseous material passing out from the aperture is subjected toa supersonic expansion.

It is a further particular preferred feature of the invention that thearc length can be varied for particular circumstances of operation, sothat for a given flow rate of material through the anode-cathode gap,and for a given potential difference across the anode cathode gap, theamount of energy available for treatment of the material may becarefully varied or "tuned". To achieve this there is preferablyprovided adjustment means for varying the said anode-cathode gap. Wherethe anode-cathode gap is an annular gap, the adjustment means isconveniently arranged to provide relative movement between the anode andthe cathode along the axis of the annular gap, the configuration of thecathode and anode combination being such that the relative movementalong the said axis produces variation in the anode-cathode gap. This isconveniently arranged for example by a combination of a cylindricalcathode and an anode having a frusto-conical surface as has been set outhereinbefore.

It is much preferred that the required movement of the arc is providedmainly or wholly by means for producing a magnetic field in the regionof the cathode-anode gap, although in alternative, less preferred,arrangements the means for producing the movement of the arc maycomprises non magnetic means, for example means for inducing swirl in agaseous medium passing through the gap between the anode and thecathode. It will be appreciated that various combinations of magneticand non-magnetic means may be used for producing the required arcmovement, the non magnetic means being used either to reinforce, or toact against, the arc movement produced by magnetic means.

Preferably the magnetic field means is arranged to produce in the regionof the cathode anode gap a magnetic field having a predominant componentat right angles to the arc. Conveniently the magnetic field is alignedalong a general longitudinal axis of the apparatus, which convenientlycomprises the axis of an annular anode cathode gap and also the axis ofa generally cylindrical cathode. Where the anode cathode gap is anannular gap, the magnetic field means conveniently comprises an annularmagnet coaxial with the said annular gap.

Where the arc moving means comprises magnetic means, it is preferredthat the magnet is spaced from the anode-cathode gap and the magneticfield is focussed to the anode-cathode gap by support means for theanode and/or cathode. Such support means can conveniently be formed ofmild steel. Such an arrangement allows a high magnetic field strength tobe maintained in the region of the arc without exposing the magnet tohigh temperatures. It is preferred that the magnet is a permanent magnetbecause, inter alia, a permanent magnet can be smaller, does not requireinsulated windings as found on an electromagnet, which windings aresubject to damage by high temperature and high voltage which maysometimes by used in striking the arc. By way of example, a preferredform of the magnetic field means may comprise a permanent annular magnetsuch as to produce along its axis when situated in air a maximum fieldin the range 250 to 350 gauss.

As has been mentioned, it is also possible to effect the requiredmovement of the arc by non-magnetic means, alone, or more convenientlyin combination with magnetic field means. Such arc moving means maycomprise means for inducing in the flow of material through theanode-cathode gap a component of motion generally transverse to thelength of the arc. Where the anode-cathode gap is an annular gap the arcmoving means may comprise means for inducing a flow of material to betreated in a direction generally tangential relative to the axis of theannular gap. Such a tangential flow will not normally be directedtangentially to the annular gap, but will be tangential to a largercylindrical chamber coaxial with the annular gap and leading thereof.Alternatively or in addition, the arc moving means may comprise swirlvanes for producing a rotary motion in the material to be treated.

By way of example, the cathode may conveniently be formed of thoriatedtungsten, tungsten, or mild steel, and the anode be formed of brass,copper or similar material.

Further by way of example of preferred ranges of dimensions which may beused, the shortest distance across the anode-cathode gap may lie in therange 250 to 350 microns.

In the same arrangements embodying the invention, there may occur acrossthe anode-cathode gap three different classes of voltage. When theapparatus is quiescent, without the arc struck, the open circuit voltageacross the anode-cathode gap may be greater than 200 volts. To start thestriking of the arc there may be required the application across the gapof a trigger voltage of the order of 20 KV. During normal runningoperation of the apparatus when the arc has been struck, the runningvoltage across the arc may be in the range 40 to 120 volts. Convenientlythere may be provided means for applying across the anode-cathode gap apotential difference such that in operation the passage of currentacross the gap by means of the arc during normal continuous operationconsumes a power in the range 400 to 1500 watts.

In preferred forms of the apparatus, the apparatus is adapted to passthe said material through the anode-cathode gap in a gaseous form. Sucha form may consist of a true gas, a vapour, or other gaseous material,for example, a suspension of metal particles in a gas. The apparatus mayconveniently include a housing providing a pressure chamber forreceiving under pressure the material to pass through the cathode-anodegap, the apparatus having an inlet for supplying material under pressureto the pressure chamber, and an outlet for passage of material out fromthe region of the anode-cathode gap after the material has passedthrough the said gap and has been treated by the arc. There may beprovided means for producing a flow of the said material through theanode-cathode gap at a flow rate in the range 60 to 500 ml/s.

In the foregoing paragraphs there have been set out a number ofpreferred and optional features of apparatus according to the presentinvention. In accordance with one particularly preferred combination ofsome of the aforesaid preferred features there is provided apparatus fortreating a flow of material by an electric arc comprising a cathode andan anode arranged for striking an arc across a gap between the cathodeand the anode and arranged to provide a pathway for flow of materialthrough the gap between the cathode and the anode, in which theanode-cathode gap is an annular gap, the anode has a region providing acircular path for the anode spot to travel around in operation, and thecathode has a region providing a circular path for the cathode root totravel around in operation, the circular paths being substantiallycoaxial, and there being provided means for producing movement of thearc struck between the cathode and the anode such that the anode spotand cathode root travel around their respective paths in the same senseand in substantially the same time with the length of the arcsubstantially constant during travel of the anode spot and cathode rootaround the said respective paths.

In accordance with another particularly preferred combination, there isprovided apparatus for treating a flow of material by an electric arccomprising a cathode and an anode arranged for striking an arc across agap between the cathode and the anode and arranged to provide a pathwayfor flow of material through the gap between the cathode and the anode,in which the anode-cathode gap is an annular gap, the anode has a regionproviding a circular path for the anode spot to travel around inoperation, and the cathode comprises a cylindrical body having, at anend thereof adjacent to the anode, a plane end face perpendicular to alongitudinal axis of the body, the end face having around the perimeterthereof an edge for striking the arc and for providing a circular pathfor the cathode root to travel around in operation, there being providedmeans for producing movement of the arc struck between the cathode andthe anode such that the anode spot and cathode root travel around theirrespective paths in the same sense and in substantially the same timewith the length of the arc substantially constant during travel of theanode spot and cathode root around the said respective paths.

There is also provided in accordance with the present invention a methodof treating a flow of material by an electric arc comprising the stepsof passing a flow of material through a gap between a cathode and ananode, striking an arc across the gap between the cathode and the anode,and producing movement of the arc struck between the cathode and theanode such as to cause the anode spot to travel around a closedcontinuous path on the anode and to cause the cathode root to travelaround a closed continuous path on the cathode, the anode spot and thecathode root being caused to travel around their respective continuousclosed paths in the same sense and in substantially the same time. Ashas been set out hereinbefore with reference to the apparatus aspect ofthe invention, it will normally be preferred that the anode spot andcathode root travel together in regular manner, as nearly as possible incorrelation with each other, and at rates of travel such that the timesof completed progressions around their respective paths are the same forcorresponding progressions around the two paths. Furthermore it is muchpreferred that in the method the movements of the anode spot and cathoderoot around their respective paths are periodic movements. In general,the various features, preferred and optional, of the invention whichhave been set out above in connection with the apparatus according tothe invention are also available as features of the method aspect of theinvention.

Preferably in accordance with the method of the invention the length ofthe arc remains substantially constant during travel of the anode spotand cathode root around their respective paths. Preferably the methodincludes the step of causing the cathode root and anode spot to movearound the said continuous closed paths in respective spaced apartparallel planes. Most preferably the said continuous closed paths arecircular paths.

In accordance with one particularly preferred feature, the method mayinclude the step of causing the cathode root to travel around its saidcontinuous closed path around the perimeter of a plane end face of acylindrical body forming the cathode, the plane end face beingperpendicular to the longitudinal axis of the cylindrical body.

In accordance with another particularly preferred feature the method mayinclude the step of striking the arc at an edge on the cathode extendingaround a closed configuration, and causing the cathode root to travelaround its said continuous closed path around the said edge.

Conveniently the method includes the step of arranging the anode-cathodegap as an annular gap and causing the materal passing through theanode-cathode gap to pass subsequently through an aperture through theanode coaxial with the annular gap. Preferably the method includes thestep of causing the material passing through the anode-cathode gap topass through an entrance to the anode aperture having a decreasing crosssection and through an exit from the anode aperture having an increasingcross section, in such a manner as to cause an expansion of the materialpassing out from the anode aperture, most preferably causing a gaseousmateral passing out from the aperture to be subjected to a supersonicexpansion.

It will normally be preferred in accordance with the invention that themethod includes the step of passing the said material through theanode-cathode gap in gaseous form.

In accordance with another particularly preferred feature the methodincludes the step of varying the said cathode-anode gap in order toimpart to the material passing through the gap a required energy toproduce a required treatment of the material.

Preferably the method includes the step of producing movement of the arcby producing a magnetic field in the region of the cathode-anode gap,conveniently a magnetic field having a predominant component at rightangles to the arc. Alternatively or in addition the method may includethe step of producing movement of the arc by inducing in the flow ofmaterial through the anode-cathode gap a component of motion generallytransverse to the length of the arc. In such latter arrangement themethod may include the steps of arranging the anode-cathode gap as anannular gap and inducing a flow of material to be treated in a directiongenerally tangential relative to the axis of the annular gap.Alternatively or in addition the method may include the steps ofarranging the anode-cathode gap as an annular gap and passing thematerial to be treated through swirl vanes for producing a rotary motionin the material to be treated.

In the foregoing paragraphs there have been set out a number ofpreferred and optional features of the method according to the presentinvention. In accordance with one particularly preferred combination ofsome of the aforesaid preferred features there is provided a method oftreating a flow of material by an electric arc comprising the steps ofpassing a flow of gaseous material through an annular gap between acathode and an anode, striking an arc across the gap between the cathodeand the anode, and producing movement of the arc struck between thecathode and the anode such as to cause the anode spot to travel around acircular path on the anode, and to cause the cathode root to travelaround a circular path on the cathode substantially coaxial with thesaid path on the anode, the anode spot and the cathode root being causedto travel around their respective paths in periodic movements in thesame sense and in the same period with the length of the arcsubstantially constant during travel of the anode spot and cathode rootaround their respective paths.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a cross-sectional view of apparatus embodying the presentinvention for producing an electrical plasma;

FIG. 2 is a side view on a similar scale of the apparatus shown insection in FIG. 1;

FIG. 3 is an end view of the apparatus shown in FIGS. 1 and 2;

FIG. 4 is a cross-section showing in magnified scale the arrangement ofan anode and cathode of the apparatus shown in FIG. 1;

FIGS. 5, 6 and 7 show alternative modifications of the anodeconfiguration shown in FIG. 1;

FIG. 8 is a cross-sectional view of a modification of the apparatus ofFIG. 1, in which gaseous feed stock is introduced by a tangential inletconduit;

FIG. 9 is a side view on a smaller scale of the apparatus shown insection in FIG. 8;

FIG. 10 is an end view of the apparatus shown in FIGS. 8 and 9; and

FIG. 11 is a cross-sectional view of a further modification of theapparatus of FIG. 1, including swirl vanes for inducing rotary motion ina gaseous feed stock passing through the apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, and moreparticularly to FIG. 1 thereof, apparatus for treating a flow of gaseousmaterial by an electric arc, commonly known as a plasma torch, isindicated generally at 11 and comprises a housing 12 provided with acathode 13 and an anode 14. The housing 12 is formed by a cylindricalhousing wall 15 supporting a cathode assembly 16 in which the cathode 13is mounted, the cathode assembly 16 being mounted in the cylindricalwall 15 by an annular insulating wall 17. (In a modified arrangement,the walls 15 and 17 may be made as a single unit of synthetic plasticsmaterial, for example nylon. This will not usually be suitable for hightemperature applications). The cylindrical wall 15 also supports theanode 14 by way of a mild steel support plate 18 which also supports anannular permanent magnet 19 positioned around the inner end of thecathode assembly 16. The sense of magnetisation of the annular magnet issuch that the poles are formed in the opposed parallel end faces of themagnet. Provision is made for optional water cooling by annular waterpassageways 20 and 21, the latter formed in an annular metal collar 22mounted on part of the cathode assembly 16 positioned exterior of thehousing 15.

The cathode 13 comprises a cylindrical rod mounted in a cathode support23 made of mild steel and forming part of the cathode assembly 16.Fixedly secured to the cathode support 23 is a lead screw 24 mounted forrotation in a threaded collar 25 and controlled by rotation of acylindrical knob 26. Rotation of the knob 26 provides precise adjustmentof the axial position of the cathode 13 along a general longitudinalaxis of the cathode and anode indicated at 27. This precise adjustmentis achieved by the combination of the thread of the lead screw and theeffect of the inclined frusto-conical face of the anode.

As shown in FIGS. 2 and 3, water supply to the water cooling passageways20 and 21 is provided by way of conduits 28 and 29, and conduits 30 and31 respectively. Inlet to the housing 12 is provided by way of an inletconduit 32 for inlet of gas or other feed stock for the plasma torch,and outlet of gas from the plasma torch is provided by an outlet conduitindicated generally at 33.

There will now be described with reference to FIGS. 1 and 4, thegeometry of a preferred configuration of cathode and anode in accordancewith the embodiment of the present invention shown in the figures. Thecathode 13 comprises a cylindrical rod with an end face 34 adjacent theanode 14 formed as a planar end face perpendicular to the axis 27 of thecathode. The anode 14 has an inwardly directed frustoconical surface 35coaxial with the axis 27 and positioned in operation around the end face34 of the cathode 13. The anode 14 is formed of a generally cylindricalbody in which the frusto-conical surface 35 is formed at one end of thecylinder. The frustoconical surface 35 is coaxial with the longitudinalaxis 27 and terminates at its inner end in an aperture 36 extendingthrough the anode cylinder 14. The aperture 36 terminates in a furtherfrusto-conical surface 37 which opens into a passageway 38 leadingthrough the outlet conduit 33 to the exterior of the housing 12. Asshown particularly in FIG. 4, the frusto-conical surface 35 is arrangedto subtend a total angle of 90° and the frusto-conical surface 37 isarranged to subtend a total angle of 120° . In other arrangements thetotal inlet angle may be less than or more than 90°. There may forexample be advantage in some arrangements in having the total inletangle less than 90° so as to give a finer degree of control over thecathode-anode spacing for a given thread of the adjustment screw 24.

Preferably the cathode 13 is formed of tungsten, thoriated tungsten,mild steel, or other similar material and the anode 14 is made of brassor other similar material. For efficient operation the metals of theanode and cathode need to be chosen to be dissimilar metals with verydifferent thermionic work functions at the potential used.

In operation, a gas or other feed stock in which the plasma is generatedis fed into the housing 12 through the inlet conduit 32, and an arc isstruck between the cathode 13 and the anode 14. The interior of thehousing 12 is maintained at a higher pressure than the pressure at theoutlet conduit 33, and the gas treated by the arc at the anode-cathodegap passes out of the reactor vessel along the aperture 36 in the anode14. The arc may be initiated by a trigger high voltage to any metal partof the apparatus or, less preferably, by direct contact initiallybetween the cathode 13 and the anode 14. Fine control over the spacingbetween the cathode 13 and the anode 14 is achieved by rotation of theknob 26.

In operation the apparatus is energised by a direct current source byconnections made to the cylindrical wall 15 of the housing 12 (positive)and to the cathode assembly 16 (negative). The device is sealed by theouter cylindrical wall 15, and isolated electrically from the cathodeassembly 16 by the wall 17 of insulating material. Preferably the plasmafeed stock in the h ousing 12 is pressurised relative to the pressure atthe outlet 38 by an amount sufficient to achieve sonic or supersonicflow through the aperture 36 in the anode 14.

In operation the arc between the cathode 13 and anode 14 is caused torotate by the magnetic field applied by means of the permanent magnet 19which is held magnetically to the plate 18 through which the magneticfield is magnetically coupled to the anode 14. The magnetic field isfocussed to the anode-cathode gap by the cathode support 23. Inalternative arrangements, an electro magnet may be used in place of thepermanent magnet 19. As has been mentioned, water cooling may besupplied in operation but in some applications this is found to beunnecessary.

Referring now particularly to FIG. 4, there will be explained the mannerof operation of the arc. Once the arc has been established, it passes asindicated at 38 across a route which is the shortest distance betweenthe cathode 13 and the anode 14. (In some circumstances the arc routemay be slightly curved and may be longer than the shortest distance, butin normal circumstances the gap will be sufficiently small for the arcto follow a substantially straight line and this will be assumed in thefollowing explanation). With the geometry shown in FIG. 4, this shortestroute of the arc 38 is approximately at right angles to thefrusto-conical surface 35 and joins the cathode 13 at a cathode root 39on the edge of the plane end surface 34 of the cathode 13. The anodespot is indicated at 40 on the frusto-conical surface 35 of the anode14. An advantage of combining the shape of a plane, perpendicular endedcylindrical cathode with a frusto-conical surface of an anode is thatfor a given position of the cathode 13 the length of the arc 38 remainsconstant for any position of the arc around the circular edge of the endsurface 34 of the cathode 13. The spacing used is generally independentof the magnetic field, and only affects the rotation rate in as far asit affects the current in the arc for a given voltage and thus theLorentz force causing the movement. Since the mild steel cathode support23 (FIG. 1) focusses the magnetic field of the magnet 19 down to thecathode-anode gap to provide a magnetic field indicated at B in FIG. 4,the effect of this magnetic field on the arc at 38 will be to make thearc rotate around the longitudinal axis 27 of the cathode-anodeassembly. This will cause both the anode spot 40 and the cathode root 39to travel around circular paths coaxial with the longitudinal axis 27.Since the arc 38 will be of constant length during this rotation themovement of the arc will be regular without any tendency to stick in oneplace and cause erosion of the cathode 13. This uniform manner ofrotation also results in greater uniformity of heating of the gaspassing through the arc. The current and magnetic field are arranged toproduce a rate of rotation of the arc such as to maintain the edge ofthe cathode at a temperature at which it will provide a plentiful supplyof electrons, yet not be deformed or eroded by being kept at a hightemperature for too long.

One factor in optimising the magnetic field strength is that the speedof arc rotation must be high enough to avoid too long a period of heattransfer to the electrodes in any particlar area, yet not so high thatinsufficient time is allowed for cooling of the electrodes, due tothermal inertia, before the next rotation of the arc. Another factorwhich has been noticed in connection with the arrangement shown in FIG.4 is believed to operate as follows. As can be seen in FIG. 4, the fieldstrength B and the electric current along the arc are not perpendicularin normal operation. If the value of the magnetic field B is increasedunduly it is believed that the field causes the arc to elongate bymoving towards a perpendicular position with respect to the magneticfield B. If, as the arc elongates, the change in arc length is suchthat, for the given potential across the anode-cathode gap, the electricfield intensity falls below that required for stable operation, then thearc would extinguish.

In selecting an optimum magnetic field for a particular arrangementfactors which sould be considered include optimisation of the rate ofrotation, and avoidance of components of arc movements that can move thearc away from its stable configuration.

It is to be appreciated that if the magnetic field were not provided,the arc would stay with the anode spot and cathode root 39 stationaryuntil either electrode was sufficiently eroded to force the arc to jumpto another position. The purpose of the rotation of the arc by themagnetic field is to maintain travel of the arc sufficiently quickly andregularly around the circular path to minimise or at least inhibiterosion from starting up. Initially the cathode is provided with a sharp90° edge. Although after long use this is eventually somewhat rounded,this occurs much less rapidly than erosion in known arrangements and toa much lesser extent. The movement of the arc around the cathode is notintended to be intermittent but is arranged to be a regular continuoustravel. If the field is not sufficiently strong to maintain this regulartravel, the arc may stick in one place and return to the conventionaloperation with a stationary or intermittently moving cathode root.

There will now be given by way of example various ranges of dimensionsand operating parameters which may be used in construction and operationof the apparatus described with reference to the figures. The diameterof the cathode 13 may conveniently be 1/8 inch (3.2 mm) and in otherarrangements may be selected from a convenient range of 1 to 5 mm. Theanode-cathode gap may conveniently be adjusted to a spacing in the range250 to 350 microns for many aplications. Other parameters may be asfolows:

    ______________________________________                                        Semiangle of surface 35                                                                           40° to 50°                                  Semiangle of surface 37                                                                           50° to 70°                                  Diameter of anode orifice 36(2r).                                                                 2.0 to 1.5 mm                                             Length of anode orifice 36(d)                                                                     3.0 to 4.0 mm                                             Change in anode-cathode gap per                                               degree of rotation of cathode                                                 (for semiangle of 45°)                                                                     2 microns                                                 Voltage across anode-cathode gap                                              during normal arc operation                                                                       40 to 120 V                                                                   (eg 80 V)                                                 Current conducted by arc during                                               normal arc operation                                                                              16 to 8 amps                                                                  (eg 10 amps)                                              Power consumed by arc during                                                  normal arc operation                                                                              1/3to 2 kW                                                                    (preferably 400 to 1500 W)                                Flow rate through gap                                                                             60 to 500 ml/s                                            Frequency of arc rotation                                                                         in the region                                                                 of 300 c/S                                                Pressure difference P.sub.1 -P.sub.2                                                              0.1 to 5                                                                      atmospheres                                               Pressure ratio P.sub.1 /P.sub.2                                                                   4 to 120                                                                      (eg 5)                                                    Strength of magnet 19 at                                                      centre in air       250 to 350                                                                    gauss                                                     ______________________________________                                    

Conveniently the electric field may be such as to produce, for example,field intensities of 10³ V/cm at 50 Volts. It is to be appreciated thatit is not appropriate to consider the state of the gas in terms oftemperatures at the anode-cathode gap of a plasma torch, but it isbelieved that at flow rates in the region f 200 ml/s the energydelivered to a feed stock gas may be 200 kJ per mole, which isequivalent to a temperature in the region of 10,000 C. for a monatomicgas.

Embodiments of the invention are primarily intended for use onpolyatomic feedstocks for example nitrogen and hydrogen. For nitrogen,using experimental data for the device, and published thermodynamicdata, temperatures have been calculated as follows (for an average arccondition):

    ______________________________________                                        N.sub.2 flow rate/(ml/sec)                                                                      Temp/K                                                      ______________________________________                                        100                5800                                                       500                3700                                                       ______________________________________                                    

For hydrogen detailed calculations have not been completed, but anestimation would place both temperatures at 2000 K. higher than N₂ ateach flow rate.

It should be noted that some devices embodying the invention may, byvirtue of the electrical efficiency, have a distribution of temperatureand species downstream of the anode. Some fraction of the feedstock gas(as determined by the volumetric efficiency) has been `heated` to a veryhigh temperature producing the atoms, radicals etc. as required, whilethe remaining component has passed through the device absorbing theminimum of power. In this way if the species generated undergosubsequent exothermic reaction and/or recombination, there will be aresultant temperature profile that will be determined by mixing patternsand rates of reaction of the subsequent process.

It will be appreciated that the embodiment of the invention shown in thefigures is distinguished from known arrangements of cathode and anode,inter alia, in that conventional cathodes have an initially pointed ordomed shape in contrast with the plane-ended perpendicular face 34 ofthe cathode rod. A dome-ended or pointed cathode is normally operatedwith the cathode root not moving in any organised manner and positionedat random positions arrived at by the arc choosing the shortest routebetween the cathode and anode. However, other shapes of cathode 13 maybe selected within the scope of the invention. For example the anglebetween the end face 34 and the cylindrical side wall (indicated at 41in FIG. 4) may be selected to be close to, but not exactly, 90°. Forexample by having the end surface 34 slightly dished, the angle betweenthe end surface 34 and the side surface 41 can be made to be less than90°. Conversely, by having the end surface 34 slightly domed, the saidangle can be arranged to be slightly greater than 90°. However, it willbe appreciated that in either of these cases, the resultant cathode isdistinguished from known cathodes in that there is provided a distinctedge or change of surface angle around the circumference of the cathoderod, and the cathode root is arranged to run around this distinct edgeas the arc moves around the axis 27. Where the angle between thesurfaces 34 and 41 is made much less than 90°, there arises thepossibility of insufficient cooling of the cathode root by heat pathsaway from the cathode root into the cathode rod, and where the anglebetween the surfaces 34 and 41 is much greater than 90°, there arisesthe possibility of the cathode root wandering from the chosen path ifthere is an insufficiently strong electric field at the distinct edgealong which the cathode root is intended to travel. In general it hasbeen found preferable to have the surfaces 34 and 41 substantially atright angles to each other.

One advantage of striking the arc at an edge on the cathode is that theeffect for the arc is the same as a point. In effect the arc is struckat a point defined by two lines in the plane of the end face and sideface of the cathode, the lines and point being rotated to define theedge. One advantage of such an edge is that the electric field lines areconcentrated by convergence at the edge.

In other forms the cathode may comprise a cylinder of metal the interiorof which may be hollow or may be filled by a different metal or by someother material.

A further advantage of the combination of a plane ended cathode with afrusto-conical anode surface is that there is provided a fine controlover the cathode-anode spacing. Axial movement of the cathode 13 byrotation of the knob 26 is resolved into a change in the cathode-anodegap which is reduced by a proportion of 1:√2 for a semi-angle of 45°.Fine adjustment is available over a longer period of operation of thedevice because of the relatively low rate of erosion compared withprevious arrangements. This fine adjustment of the cathode-anode spacingallows fine tuning of the electric field intensity (the applied voltagedivided by the length of the arc) of the plasma torch to achieveparticularly required characteristics of plasma from a given plasmafeedstock. For a given flow rate of gas through the anode-cathode gap,the ability to control the electric field intensity precisely enablesthe choice of excitation of species to be generated. Such tuning shouldalso taken into account other factors. An appropriate anodeconfiguration, i.e. a particular surface to volume ratio in the anodeaperture, and particular diameter of outlet nozzle, can be chosen togive a required plasma for any pressure of the region into which thehousing 12 discharges. This gives a greater ability to operate intosubsequent low pressure systems than is available with conventionalplasma torches. In many respects the higher back pressure in the housing12 relative to the region into which the torch operates, the better isthe effect on the plasma generation efficiency.

By considering the effect of the geometry of the anode on the finetuning by the knob 26, it will be seen that other inlet angles may beadvantageous. For instance, if the anode has an inlet semi-angle of α,then the fine tuning control obtained by knob 26 is (sinα)/x perrevolution, x being the number of threads per inch of component 24. Fora given value of x (eg 26 or 40 TPI) there would be a fine tuningadvantage on the arc distance by choosing α less than 45°, eg asemi-angle of 30°.

The importance of fine tuning in operation is that for a given value ofapplied voltage and flow rate of feedstock, the tuning enables controlover the field intensity (defined here as applied voltage/arc length)such that it is possible to control the excitation of the atoms/radicalsgenerated. This has been found to be the case by examination of theemission spectra of the nitrogen recombination spectra.

Because the fine tuning of the arc for selective excitation depends onthe volumetric flow rate of gas through the gap (as well as otherfactors) it is important when such tuning is desired, that fluctuationsin the downstream pressure P₂ (FIG. 4) do not produce changes in theupstream pressure P₁. When the upstream pressure P₁ is sufficient forthe velocity of sound to be reached in the anode orifice, fluctuationsin the downstream pressure will not feed back and will not affect theupstream pressure. When the pressure ratio P₁ /P₂ is above the criticalvalue to achieve the speed of sound in the anode orifice, pressurefluctuations in the pressure P₂ in no way affect P₁ and therefore do notaffect the stable operation of the arc. This effect is important wherethe anode orifice opens into a region liable to pressure variations, forexample in a pulsating exhaust system.

Another factor which needs to be considered is the surface to volumeratio of the anode. Referring to FIG. 4 the following may be defined:

P₁ /P₂ =pressure ratio (or expansion ratio)

Q_(A) =heat lost from anode spot of arc to anode

Q_(R) =heat lost from cylindrical surface of anode orifice due torecombination within orifice.

Q_(R) is a function of d and S/V

(Both Q_(A) and Q_(R) have to be dissipated by water or alternativecooling, and lower the electrical efficiency).

r=orifice radius

S/V=surface to volume ratio of orifice (2/r²)

β=recombination coefficient.

β is a function of S/V and P₁ /P₂ and is very important for electricalefficiency measured by atoms entering the P₂ region.

P₂ is dictated by the operating condition (eg atmospheric, 60 Torr etc)which sets a range of P₁ /P₂ that may be used depending on operatingcriteria.

Generally one wants to optimise P₁ /P₂ as large as possible to increasedissocation and to increase lifetime in the P₂ region. However this mustbe done while minimising Q_(A), Q_(R), S/V,d and β, while maximising rto the point where P₁ /P₂ may be maintained by the anode. (If d is toosmall, anode erosion is severe).

By way of example, dimensions which have given good results for the wideapplication are:

r=1 mm

P₁ /P₂ =4 to 120

S/V=2

d=3 mm

These are suitable for nitrogen and in the pressure range 5 Torr to 1atmosphere. Under these conditions, ground state atom generation rateshave been found to be 10¹⁹ to 10²¹ atoms/sec and electrical efficiencyabout 50%. β was in the range 0.4 to 0.5.

In one particular example of application the following anode dimensionswere used:

r=1 mm

P₁ /P₂ =16

S/V=2

d=2.9 mm

Where the feedstock gas was nitrogen, the device produced 10²⁰ groundstate (4s) atoms per second with an electrical efficiency of 53%.

Another preferred feature of the arrangement shown in the figures isthat the magnet 19 can be a bonded ferrite permanent magnet which can beprovided in an insulating casing. This allows minaturisation of theentire apparatus giving the possibility of use in various applicationspreviously not available for conventional plasma torches. The use ofsuch a type of magnet is made convenient by the focussing of themagnetic field down to the cathode-anode gap as has been described. Insome applications of such a plasma torch, there is a danger that thetemperature of the magnet will rise above the Curie point withconsequent destruction of the magnet. It is therefore important to placethe magnet in a relatively cool place in the assembly and then to focusthe magnetic field down to the required position at the cathode-anodegap. The advantage of a permanent magnet is that there is no melting ofthe insulation which is present with the wiring of an electromagnet.Another advantage of using a permanent magnet rather than electromagnetis that if it is attempted to start the arc using a Tesla coil whenusing an electromagnet, the insulation on the windings of theelectromagnet can breakdown leading to a short circuit and failure ofthe magnet.

It will be appreciated however that it is not essential for steelsupport means to be provided for focussing the magnetic field. Theapparatus can be made to operate satisfactorily with a brass or aluminumcathode support material, provided that the magnet strength is increasedcorrespondingly.

The effect of the magnetic field in rotating the arc also has abeneficial effect when required in producing swirl and turbulence in thegaseous stream leaving the outlet conduit 33.

There will now be given by way of example particular operatingparameters which may be used for applications of the apparatus shown inFIGS. 1 to 4.

EXAMPLE 1 Generation of N atoms for exhaust gas cleaning

    ______________________________________                                        Feedstock material  nitrogen gas                                              Cathode rod diameter                                                                              1/8 inch (3.2 mm)                                         Anode-cathode gap   300 microns                                               Voltage across gap  80 volts                                                  Current through arc 10 amps                                                   Power consumed      800 watts                                                 Flow rate           100 ml/s at S.T.P.                                        Pressure drop (P.sub.1 -P.sub.2)                                                                  3 atmospheres                                             Strength of field of                                                          magnet 19 (at centre                                                          in air)             300 gauss                                                 ______________________________________                                    

EXAMPLE 2 Generation of N atoms for exhaust gas cleaning

    ______________________________________                                        Feedstock material  nitrogen gas                                              Cathode rod diameter                                                                              1/8 inch (3.2 mm)                                         Anode-cathode gap   300 microns                                               Voltage across gap  80 volts                                                  Current through arc 10 amps                                                   Power consumed      800 watts                                                 Flow rate           500 ml/s                                                  Pressure ratio P.sub.1 /P.sub.2                                                                   3                                                         Strength of field of                                                          magnet 19 (at centre                                                          in air)             300 gauss                                                 Anode orifice radius                                                          r                   1.25 mm                                                   Anode orifice length                                                          d                   3.0 mm                                                    ______________________________________                                    

EXAMPLE 3 Generation of plasma jet for cutting purposes

    ______________________________________                                        Feedstock material  H.sub.2                                                   Cathode rod diameter                                                                              1/8 inch (3.2 mm)                                         Anode-cathode gap   300 microns                                               Voltage across gap  50 V                                                      Current through arc 30 A                                                      Power consumed      1500 W                                                    Flow rate           300 to 400 ml/s                                                               at S.T.P.                                                 Pressure drop (P.sub.1 -P.sub.2)                                                                  4 atmospheres                                             Strength of field of                                                          magnet 19 (at centre                                                          in air)             250 gauss                                                 ______________________________________                                    

EXAMPLE 4 Plasma jet as continuous H atom source for a jet ignitor

    ______________________________________                                        Feedstock material  Hydrogen gas                                              Cathode rod diameter                                                                              3 mm                                                      Anode-cathode gap   250 microns                                               Voltage across gap  60 Volts                                                  Current through arc 9 amps                                                    Power consumed      540 watts                                                 Flow rate           500 ml/sec                                                Pressure Ratio P.sub.1 /P.sub.2                                                                   3                                                         Strength of magnet 19                                                         (at centre in air)  300 gauss                                                 Anode orifice radius r                                                                            1.0 mm                                                    Anode orifice length d                                                                            3.5 mm                                                    ______________________________________                                    

Advantages which may arise in the use of the device operating in thismode are for example the ability to ignite kerosine sprays in coldenvironments (this was previously difficult even with surface dischargeplugs) or the re-ignition of a high altitude jet engine after`flame-out`. The working flow rate range that the device can handleusing hydrogen is from 100 to 700 ml/sec.

Various advantages arise in the use of devices such as have beendescribed and give rise to a number of particularly useful applicationsof such a torch. Preferred embodiments of the invention can be arrangedto enable an electrical plasma to be produced from a gas or other feedstock continuously, at a low power input, and without the necessity forseeding or use of auxiliary gases such as argon. These gases arecommonly used to improve arc stability but the presence of a monatomicgas such as argon necessarily reduces the efficiency in known plasmatorches. Preferred embodiments of the invention can also be arranged tooperate in many instances without any water cooling. The presence ofwater cooling necessarily implies a lower efficiency, since part of theenergy put into the device is being conducted away rather than beingused in the production of plasma.

In many instances the plasma torch will be coupled to a subsequentsystem in which chemical reactions are required. In such a case theplasma feedstock passes into the rotating arc region and issues from theanode into the downstream region where the required chemical process istaking place. Such a device may be used, with suitable choice of gaseousand other feedstocks to produce atoms, ions, electrons, radicals orother chemical entities which are subsequently reacted downstream of theapparatus in a chemical process that involves the entities generatedwithin the device. The swirl which may be introduced into the plasmaemerging from the anode (as has been mentioned) is advantageous inensuring mixing of the entities generated by the device with thematerial to be processed subsequently downstream. By way of examples offeedstock, embodiments of the invention can be arranged to operatecontinuously on nitrogen or hydrogen inputs and may be used, if desired,as a source of nitrogen or hydrogen atoms. These atoms may be used, forexample, for chemical processes such as the following.

In pollution removal use may be made of the reaction:

    N+NO→N.sub.2 +O

Here nitric oxide is removed by nitrogen atoms producing molecularnitrogen and an oxygen atom that may be used for further processing. Aplasma torch embodying the invention may be used to inject these atomsfor example into the exhaust gases from a combustion plant, thuslowering the nitric oxide emission from such a plant.

Another example of pollution removal is in soot removal by oxidation orelectrical modification of the soot forming process by material injectedfrom the plasma torch. This will increase the rate of oxidation of thesoot and/or modify its electrical surface properties and henceaggregation thus facilitating its removal from combustion products.Pollutants such as oxides of nitrogen and soot can be destroyed by theinjection of the output from a nitrogen plasma.

A further example of use of material produced by a plasma torchembodying the invention is in the field of combustion enhancement byinjection of appropriate chemical entities from such a plasma torch. Forexample hydrogen or nitrogen atoms may be used to increase flamestability, cause ignition and give faster combustion in fuel-leanmixtures with less pollution.

Another example of use of material produced by a plasma torch embodyingthe invention is in synthetic reactions. The injection of theappropriate chemical entities from the device may be used as a syntheticroute to a number of important small molecules as well as polymers, forexample the production of hydrogen cyanide by reaction of nitrogen atomsproduced by a plasma torch embodying the invention with an appropriatehydrocarbon, for example methane.

The invention has applications to molecular beams as well as to lasers.The invention also finds use as an ignition source for a jet engine,where it may operate intermittently on a hydrocarbon or even on watervapour.

These examples are not intended to be exhaustive, but are merely givento illustrate some of the potential uses of embodiments of theinvention.

Referring again to advantages which can be obtained from preferredembodiments of the invention, it is found that in appropriateembodiments, virtually any gas or vapour system can be readilydissociated into atoms, radicals and electrons with sufficient atomdensity to allow efficient reaction. For example 10²⁰ N atoms/second canbe generated. The efficiency of operation can be made much higher thanpreviously in that more than 50% of the electrical energy supplied tothe apparatus can be utilised in breaking bonds and the gas stream atthe outlet can be made to remain relatively cool. Embodiments can run ata power of 500 W which is much lower than conventional plasma torches.The overall construction allows a simple and cheap plasma torch to beproduced, the bulk of the construction materials being mild steel andbrass and in some cases plastics material. As has been mentioned, aplasma torch embodying the invention can be made to be finely tunable sothat atoms can be generated with a required internal electronic state.This can lead to specificity in reactions and yields approaching 100%.Thus in some arrangements it is possible to provide a continuouslyoperating plasma jet which combines a high efficiency with highstability over a wide range of flows without requiring the use of anadded monatomic gas such as argon.

Referring again to the accompanying drawings there are shown in FIGS. 5,6 and 7 modifications of the anode shown in FIGS. 1 to 4. In FIGS. 5, 6and 7, elements corresponding to elements in the preceding figures willbe referred to by like reference numerals. Similar notation will be usedin other following figures. In FIG. 5, the anode is of the same generalshape as that shown in FIG. 1, but the length of the anode aperture 36is reduced. In FIG. 6 the cylindrical anode aperture has been omittedand the surfaces 35 and 37 meet directly. In FIG. 7, the frusto-conicalexit from the anode aperture has been omitted and replaced by a flat,plane surface.

Referring to FIGS. 8, 9 and 10, there will now be described a furthermodification of the embodiment of FIGS. 1 to 4. In this modification itis possible with the same electrode configuration to cause the arc torotate by means of the manner of injection of the feed stock gas intothe housing 15. Conveniently this may be carried out by positioning aninlet 32 in such a manner as to cause a tangential flow of the feedstock gas into the housing 12. The object of such an inlet is that atleast a component of the gas flow shall impart an angular momentum tothe gaseous material within the housing such as to cause the arc torotate.

This means of causing the arc to rotate by a rotary movement of thegaseous feed stock can be used either alone, in place of the magneticfield described above, or in combination with such a magnetic field.

When the gas flow induced rotation of the arc is used alone in theabsence of a magnetic field, some of the advantages set out above do notarise, because of the decreased relative motion between the arc and thegas. When a magnetic field induces rotation of the arc, the arc isforced to rotate through the gas thereby causing more uniform heating ofthe gas. In the case where gas flow induced rotation of the arc is usedalone, the relative velocity between the arc and the gas will be less,so that there may be tendency to have a greater temperature distributionthrough the product gases.

Even where there is produced magnetic means for rotating the arc, it isbelieved that some temperature distribution will exist in the gasesdownstream of the arc, and that this will depend on the material passingthrough the arc. Calculations of volumetric efficiency and electricalefficiency are thought to show such temperature inhomogeneities whichare believed to arise because there are present atoms and ions in arelatively cool gas expanding from the anode aperture. The downstreaminhomogeneities are believed to come from areas of recombinationdownstream in the absence of reactants (i.e. atoms with atom, and ionwith electron), and from heats of reaction where atoms reactexothermically.

It is to be appreciated that the aspect of the invention in whichmovement of the arc is produced by gas flow is not limited to the use ofa tangential input gas flow. The required movement can be obtained byother means which induce swirl in the gaseous medium in which the arc isstruck. In FIG. 11 there is shown a modification of the apparatus ofFIG. 1 in which arc movement is obtained by use of swirl vanes 40 whichact upon the gas flow in the space between the cathode support 23 andthe forward part of the anode support 18.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the amended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and described to be secured by Letters Patent ofthe United States is:
 1. Apparatus for treating a flow of material by anelectric arc comprising:a cathode and an anode arranged for striking anarc across a gap between the cathode and the anode and arranged toprovide a pathway for flow of material through the gap between thecathode and the anode, means for producing movement of the arc aroundthe anode-cathode gap, and adjustment means for selectively increasingand decreasing the distance across the anode-cathode gap by rearward andforward axial movement of the cathode for fine control of the internalenergy imparted to material passing through the anode-cathode gap, thecathode being a substantially non-consumable cathode having an edge atan end thereof opposite the anode for providing a continuous closed pathfor the cathode root to travel around in operation, the anode having aninwardly tapering frusto-conical surface arranged opposite the said edgeon the cathode in a position such as to provide on the frusto-conicalsurface a continuous closed path for the anode spot to travel around inoperation, the distance across the anode-cathode gap being sufficientlysmall for the arc channel to be substantially straight between thecathode root and anode spot in normal operation during the said movementof the arc around the gap, and the angles of inclination of the surfacesforming the said edge on the cathode being such relative to each otherand relative to the frusto-conical surface of the anode as to preventthe cathode root wandering from the edge along which it is intended totravel during the said movement of the arc around the anode-cathode gap,the arrangement being such that the anode spot and cathode root travelaround their respective paths in the same sense and in substantially thesame time with the arc free from uncontrolled changes in arc lengthduring travel of the anode spot and cathode root around the saidrespective paths.
 2. Apparatus according to claim 1 in which theconfiguration of the anode and cathode is such that axial movement ofthe cathode by the said adjustment means produces change in the distanceacross the anode-cathode gap without change in the geometricalrelationship between the arc and the surfaces of the anode and cathodein the region of the anode spot and cathode root, respectively. 3.Apparatus according to claim 1 in which the configuration of the cathodeand the anode and the distance across the anode-cathode gap are suchthat thoughout the movement of the arc around the anode-cathode gap, thearc channel lies substantially at right angles to a tangent to the saidedge of the cathode taken at the cathode root.
 4. Apparatus according toclaim 1 in which the distance across the anode-cathode gap is, inoperation, less than 350 microns.
 5. Apparatus according to claim 1 inwhich the distance across the anode-cathode gap lies, in operation, inthe range 250 to 350 microns.
 6. Apparatus according to claim 1 in whichthe configuration of the cathode and the anode and the arrangement ofthe arc moving means are such that in operation the length of the arcremains substantially constant during travel of the anode spot andcathode root around their respective paths.
 7. Apparatus according toclaim 1, in which the said edge of the cathode is a circular edge. 8.Apparatus according to claim 7 in which the cathode comprises acylindrical body having, at an end thereof adjacent to the anode, aplane end face perpendicular to a longitudinal axis of the body, thesaid continuous closed path of the cathode root being provided aroundthe perimeter of the plane end face of the cylindrical body. 9.Apparatus according to claim 7, in which the cathode has a diameter inthe range 1 to 55 mm.
 10. Apparatus according to claim 1 in which theedge is defined by surfaces leading to each other in the region of theedge at an angle of substantially 90°.
 11. Apparatus according to claim1 in which the said edge which defines the said continuous closed pathfor the cathode root comprises an edge of a cathode constituted by asolid rod.
 12. Apparatus according to any of claims 1 to 6 in which theanode has an aperture through the anode at the inner end of saidfrusto-conical surface for passage of material which has passed throughthe said anode-cathode gap.
 13. Apparatus according to claim 12 in whichthe anode is shaped to provide in the region of the cathode an entranceto the aperture having a decreasing cross section and to provide at theother end of the aperture an exit having an increasing cross section,the arrangement being adapted to provide in operation an expansion ofmaterial passing out from the anode aperture.
 14. Apparatus according toclaim 12 in which the anode has a further frusto-conical surface formedon a side of the anode remote from the cathode and coaxial with theaperture through the anode.
 15. Apparatus according to claim 14 in whichthe said further frusto-conical surface has a semi-angle in the range50° to 70°.
 16. Apparatus according to claim 15 in which the saidfurther frusto-conical surface has a semi-angle of substantially 60°.17. Apparatus according to claim 1 in which the semi-angle of the saidfrusto-conical surface facing the cathode is in the range of 30° to 50°.18. Apparatus according to claim 17 in which the semi-angle of the saidfrusto-conical surface facing the cathode is substantially 45°. 19.Apparatus according to any of claims 1 to 6 in which the arrangement issuch that in operation gaseous material passing out from the aperture issubjected to a supersonic expansion.
 20. Apparatus according to any ofclaims 1 to 6 in which the arc moving means is such that in operationthe movements of the anode spot and the cathode root around theirrespective paths are periodic movements.
 21. Apparatus according to anyof claims 1 to 6 in which the arc moving means comprises means forproducing a magnetic field in the region of the cathode-anode gap. 22.Apparatus according to claim 21 in which the magnetic field means isarranged to produce in the region of the cathode-anode gap a magneticfield having a predominant component at right angles to the arc. 23.Apparatus according to claim 21 in which the anode-cathode gap is anannular gap and the magnetic field means comprises an annular magnetcoaxial with the said annular gap.
 24. Apparatus according to claim 21in which the said magnetic field means comprises a permanent magnetspaced from the cathode-anode gap, and the magnetic field is focussed tothe anode-cathode gap by supporting means for the cathode and/or anode.25. Apparatus according to claim 24 in which the magnet comprises apermanent annular magnet such as to produce along its axis when situatedin air a field in the range 250 to 350 gauss.
 26. Apparatus according toany of claims 1 to 6 in which the arc moving means comprises means forinducing in the flow of material through the anode-cathode gap acomponent of motion generally transverse to the length of the arc. 27.Apparatus according to claim 26 in which the anode-cathode gap is anannular gap and in which the arc moving means comprises means forinducing a flow of material to be treated in a direction generallytangential relative to the axis of the annular gap.
 28. Apparatusaccording to claim 29, in which the anode-cathode gap is an annular gapand in which the said arc moving means comprises swirl vanes forproducing a rotary motion in the material to be treated.
 29. Apparatusaccording to any of claims 1 to 6 in which the cathode is formed ofthoriated tungsten, tungsten or mild steel, and the anode is formed ofbrass, copper or silver.
 30. Apparatus according to any of claims 1 to 6including means for supplying across the anode-cathode gap during normalcontinuous operation of the apparatus a potential difference in therange 40 to 120 volts.
 31. Apparatus according to any of claims 1 to 6including means for supplying across the anode-cathode gap a potentialdifference such that in operation the passage of current across the gapby means of the arc during normal continuous operation consumes a powerin the range 400 to 1500 watts.
 32. Apparatus according to any of claims1 to 6 including means for producing a flow of the said material throughthe anode-cathode gap at a flow rate in the range 60 to 500 ml/s. 33.Apparatus according to any of claims 1 to 6 including a housingproviding a pressure chamber for receiving under pressure the materialto pass through the cathode-anode gap, the apparatus having an inlet forsupplying material under pressure to the pressure chamber, and an outletfor passage of material out from the region of the anode-cathode gapafter the material has passed through said gap and has been treated bythe arc.
 34. Apparatus according to any of claim 1 to 6 in which theapparatus is adapted to pass the said material through the anode-cathodegap in a gaseous form.
 35. A method of treating a flow of material by anelectric arc, comprising:passing a flow of material through a gapbetween a substantially non-consumable cathode having an edge forstriking the arc and an anode having an inwardly tapering frusto-conicalsurface positioned opposite the edge of the cathode, the angles ofinclination of the surfaces forming the said edge of the cathode beingarranged to be such relative to each other and relative to thefrusto-conical surface of the anode as to prevent the cathode rootwandering from the said edge during normal operation, striking an arcacross the anode-cathode gap between the edge of the cathode and thefrusto-conical surface of the anode, selectively increasing anddecreasing the distance across the anode-cathode gap by axial movementof the cathode to control the internal energy imparted to the materialpassing through the anode-cathode gap and adjusting the cathode to aposition in which the distance across the anode-cathode gap issufficiently small for the arc channel to be substantially straightbetween the cathode root and anode spot during normal operation, andproducing movement of the arc around the anode-cathode gap such that thecathode root travels along a continuous closed path around the said edgeof the cathode and the anode spot travels along a continuous closed patharound the said frusto-conical surface, and the cathode root and anodespot travel around their respective paths in the same sense and insubstantially the same time with the arc free from uncontrolled changesin arc length during the travel of the anode spot and the cathode root.36. A method according to claim 35 including varying the anode-cathodegap by producing changes in the distance across the anode-cathode gapwithout change in the geometrical relationship between the arc and thesurfaces of the anode and cathode in the region of the anode spot andcathode root respectively.
 37. A method according to claim 35 includingstriking the arc in such a manner that throughout the movement of thearc around the anode-cathode gap, the arc channel lies substantially atright angles to a tangent to the edge of the cathode taken at thecathode root.
 38. A method according to claim 35 including striking thearc across an anode-cathode gap having a distance less than 350 micronsacross the gap.
 39. A method according to claim 35 including strikingthe arc across an anode-cathode gap having a distance across the gaplying in the range 250 to 350 microns.
 40. A method according to claim35 in which the length of the arc is kept substantially constant duringtravel of the anode spot and cathode root around their respective paths.41. A method according to claim 35 in which the said edge of the cathodeis a circular edge.
 42. A method according to claim 35 including thestep of striking the arc at an edge defined by surfaces leading to eachother in the region of the edge at an angle of substantially 90 °.
 43. Amethod according to any of claims 35 to 37 including the step of causingthe cathode root to travel along its said continuous closed path aroundthe perimeter of a plane end face of a cylindrical body forming thecathode, the plane end face being perpendicular to the longitudinal axisof the cylindrical body.
 44. A method according to any of claims 35 to39 including the step of arranging the anode-cathode gap as an annulargap and causing the material passing through the anode-cathode gap topass subsequently through an aperture through the anode coaxial with theannular gap.
 45. A method according to claim 44 including the step ofcausing the material passing through the anode-cathode gap to passthrough an entrance to the anode aperture having a decreasing crosssection and through an exit from the anode aperture having an increasingcross section, in such a manner as to cause an expansion of the materialpassing out from the anode aperture.
 46. A method according to claim 45including the step of causing a gaseous material passing out from theaperture to be subjected to a supersonic expansion.
 47. A methodaccording to any of claims 35 to 39 including the step of varying thesaid cathode-anode gap in order to impart to the material passingthrough the gap a required internal energy to produce a requiredtreatment of material in a subsequent downstream reaction.
 48. A methodaccording to any of claims 35 to 39 including the steps of moving theanode spot around its closed continuous path in a periodic movement, andmoving the cathode root around its closed continuous path in a periodicmovement.
 49. A method according to any of claims 35 to 39 including thestep of producing movement of the arc by producing a magnetic field inthe region of the cathode-anode gap.
 50. A method according to claim 49including the step of producing in the region of the cathode-anode gap amagnetic field having a predominant component at right angles to thearc.
 51. A method according to any of claims 35 to 39 including the stepof producing movement of the arc by inducing in the flow of materialthrough the anode-cathode gap a component of motion generally transverseto the length of the arc.
 52. A method according to claim 51 includingthe steps of arranging the anode-cathode gap as an annular gap andinducing a flow of material to be treated in a direction generallytangential relative to the axis of the annular gap.
 53. A methodaccording to claim 51 including the steps of arranging the anode-cathodegap as an annular gap and passing the material to be treated throughswirl vanes for producing a rotary motion in the material to be treated.54. Apparatus for treating a flow of material by an electric arccomprising:a cathode and an anode arranged for striking an arc across agap between the cathode and the anode and arranged to provide a pathwayfor flow of material through the gap between the cathode and the anode,means for maintaining the arc by supplying across the anode-cathode gapa potential difference in the range 40 to 120 volts with a powerconsumption in operation in the range 400 to 1500 watts, means forproducing movement of the arc around the anode-cathode gap, andadjusting means for selectively increasing and decreasing the distanceacross the anode-cathode gap without change in the geometricalrelationship between the arc and the surfaces of the anode and cathodeby rearward and forward axial movement of the cathode for fine controlof the internal energy imparted to the material passing through theanode-cathode gap, the cathode being a substantially non-consumablecathode having an average width in the range 1 to 5 mm and having anedge at an end thereof opposite the anode for providing a continuousclosed path for the cathode root to travel around in operation, theanode having an inwardly tapering frusto-conical surface arrangedopposite the said edge on the cathode in a position such as to provideon the frusto-conical surface a continuous closed path for the anodespot to travel around in operation, the distance across the anodecathode gap lying in the range 250 to 350 microns and being sufficientlysmall for the arc channel to be substantially straight between thecathode root and anode spot in normal operation during the said movementof the arc around the gap, the said frusto-conical surface of the anodehaving a semi-angle in the range 30° to 50° and the angles ofinclination of the surfaces forming the said edge on the cathode beingsuch relative to each other and relative to the frusto-conical surfaceof the anode as to prevent the cathode root wandering from the edgealong which it is intended to travel during the said movement of the arcaround the anode cathode gap, the arrangement being such that the anodespot and cathode root travel around their respective paths in the samesense and in substantially the same time with the arc free fromuncontrolled changes in arc length during travel of the anode spot andcathode root around the said respective paths.
 55. Apparatus fortreating a flow of material by an electric arc comprising:a cathode andan anode arranged for striking an arc across a gap between the cathodeand the anode and arranged to provide a pathway for flow of materialthrough the gap between the cathode and the anode, means for producingmovement of the arc around the anode cathode gap, and adjustment meansfor selectively increasing and decreasing the distance across the anodecathode gap without change in the geometrical relationship between thearc and the surfaces of the anode and cathode by rearward and forwardaxial movement of the cathode for fine control of the internal energyimparted to material passing through the anode cathode gap, the cathodebeing a substantially non-consumable cathode having a circular edge atan end thereof opposite the anode for providing a continuous closed pathfor the cathode root to travel around in operation, the anode having aninwardly tapering frusto-conical surface arranged coaxially with andopposite to the said circular edge on the cathode in a position such asto provide on the frusto-conical surface a continuous closed circularpath for the anode spot to travel around in operation, the distanceacross the anode cathode gap being sufficiently small for the arcchannel to be substantially straight between the cathode root and anodespot in normal operation during the said movement of the arc around thegap, with the arc channel lying substantially at right-angles to atangent to the said edge of the cathode taken at the cathode root, andat right-angles to the frusto-conical surface of the anode, the saidcircular edge being defined by a right circular cylindrical outersurface of the cathode meeting a plane end surface of the cathode atright-angles to the plane end surface, and the said frusto-conicalsurface of the anode being coaxial with the cylindrical cathode andhaving a semi-angle of 45°, whereby the angles of the inclination of thesurfaces forming the said edge on the cathode are such relative to eachother and relative to the frusto-conical surface of anode so as toprevent the cathode root wandering from the edge along which it isintended to travel during the said movement of the art around the anodecathode gap, the arrangement being such that the anode spot and cathoderoot travel together around their respective paths with the arc lengthsubstantially constant during the travel of the anode spot and cathoderoot.
 56. A method of treating a flow of material by an electric arc,comprising:passing a flow of material through a gap between asubstantially non-consumable cathode and an anode having an inwardlytapering frusto-conical surface of semi-angle in the range 30° to 50°positioned opposite the end of the cathode, striking across theanode-cathode gap between an edge of the cathode and the frusto-conicalsurface of the anode an arc which is substantially straight between thecathode root and anode spot, the angles of inclination of the surfacesforming the said edge on the cathode being arranged to be such relativeto each other and relative to the frusto-conical surface of the anode asto prevent the cathode root wandering from the said edge during normaloperation, maintaining the arc during normal operation by supplyingacross the anode-cathode gap a potential difference in the range 40 to120 volts with a power consumption in the range 400 to 1500 watts,producing movement of the arc around the anode-cathode gap such that thecathode root travels along a continuous closed path around the said edgeof the cathode and the anode spot travels along a continuous closed patharound the said frusto-conical surface, and the cathode root and anodespot travel around their respective paths in the same sense and insubstantially the same time with the arc free from uncontrolled changesin arc length during the travel of the anode spot and cathode root, andselectively increasing and decreasing the distance across theanode-cathode gap without change in the geometrical relationship betweenthe arc and the surfaces of the anode and cathode by rearward andforward axial movement of the cathode to control the internal energyimparted to the material passing through the anode-cathode gap, thedistance across the anode-cathode gap being adjusted so as to lie in therange 250 to 350 microns in normal operation.
 57. A method of treating aflow of material by an electric arc, comprising:passing a flow ofmaterial through a gap between a substantially non-consumable cathodehaving a circular edge for striking the arc and an anode having aninwardly tapering frusto-conical surface positioned coaxially with andopposite to the edge of the cathode, the said circular edge beingdefined by a right circular cylindrical outer surface of the cathodemeeting a plane end surface of the cathode at right angles to the planeend surface, and the said frusto-conical surface of the anode beingcoaxial with the cylindrical cathode and having a semi-angle of 45°,whereby the angles of inclination of the surfaces forming the said edgeon the cathode are arranged to be such relative to each other andrelative to the frusto-conical surface of the anode as to prevent thecathode root wandering from the said edge during normal operation,striking across the anode-cathode gap between the edge of the cathodeand the frusto-conical surface of the anode an arc which issubstantially straight between the cathode root and anode spot duringnormal operation and which lies substantially at right-angles to atangent to the said edge of the cathode taken at the cathode root and atright-angles to the frusto-conical surface of the anode, producingmovement of the arc around the anode-cathode gap such that the cathoderoot travels along a continuous closed circular path around the saidedge of the cathode and the anode spot travels along a continuous closedcircular path around the said frusto-conical surface, and the cathoderoot and anode spot travel around their respective paths in the samesense and in substantially the same time with the arc lengthsubstantially constant during the travel of the anode spot and cathoderoot, and selectively increasing and decreasing the distance across theanode-cathode gap without change in the geometrical relationship betweenthe arc and the surfaces of the anode and cathode by rearward andforward axial movement of the cathode to control the internal energyimparted to the material passing through the anode-cathode gap.